Deposit removal method

ABSTRACT

A deposit removal method including a first process of stripping at least part of a deposit that has deposited on inner walls of a reaction chamber and/or a surface of components located inside the reaction chamber where a deposited film is formed from the inner walls of the reaction chamber and/or the surface of components located inside the reaction chamber; and a second process of physically removing the stripped deposit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a deposit removal method for removing a deposit that has deposited in a reaction chamber when a deposited film is formed in the reaction chamber by plasma CVD, thermal CVD, or sputtering.

2. Description of the Related Art

In a plasma CVD method, a starting material gas is introduced in a reaction chamber, the reaction chamber is depressurized with an evacuation pump, direct current power, high-frequency power, or microwave power is applied, the starting material gas is ionized, dissociated, and excited to obtain plasma, and a deposited film is formed on a substrate. The conventional plasma CVD method uses parallel flat electrodes and employs a glow discharge or an RF discharge using high frequency.

In addition to the discharge method using parallel flat electrodes, methods are used by which a compound gas is decomposed and deposition is induced by thermal energy. Examples of methods using thermal energy include a Hot Wall method by which a gas with a comparatively low decomposition temperature, such as Si₂H₆, is used as a starting material and the gas is decomposed by heating the reaction chamber itself, and a thermal CVD method in which a similar effect is obtained by heating a substrate. Furthermore, there is a Hot Wire CVD method by which thin film formation is performed by using a metal filament such as a tungsten filament that is heated to a temperature equal to or higher than a melting point of silicon crystal. A photo-CVD method is also used by which a starting material is decomposed and a deposited film is formed by irradiating the substrate surface with light such as ultraviolet radiation.

In a CVD device, as a desired thin silicon film is formed on a substrate by the CVD reaction, an unnecessary deposit such as the thin silicon film or polysilane is unavoidably deposited on the inner walls of the reaction chamber or surface of components located inside the reaction chamber, such as a high-frequency electrode. The amount of adhered deposit increases with the film formation time, and the deposit that has unstably adhered can be separated or scattered by the starting material gas flow or under the effect of vibrations created by an evacuation pump. This separated or scattered deposit can reach the substrate and cause defects in the desired thin silicon film.

Furthermore, in a plasma CVD device, because the deposit that has adhered to the surface of a high-frequency electrode is an electrically resistant component, the electric resistance in plasma changes and the plasma state can change.

Therefore, once the predetermined film formation time in the CVD device elapses, the device is typically disassembled and cleaned manually. In such a case, in order to remove not only the deposit that has unstably adhered to the inner walls of the reaction chamber or components located inside the reaction chamber, but also the deposit that has strongly adhered, mechanical cleaning, for example, by sandblasting is also used. Cleaning using an alkali solution is also performed.

However, a method of manually disassembling the CVD device and cleaning requires a long time for disassembling, cleaning, and again assembling the device and also involves a large labor cost. Moreover, in some cases, a cleaning gas (for example, a fluorine-including gas such as CF₄ or ClF₃) or a portion of NaOH (alkali reagent) is adsorbed by the internal components of the reaction chamber as a result of cleaning. Where the cleaning gas or reagent is thus adsorbed by the internal components of the reaction chamber, when the deposited film formation is restarted, the gas or reagent can penetrate as contamination into the deposited film and degrade the film properties. Therefore, an additional process is required for removing completely the gas or reagent (for example, a purging process). As a result, the downtime of the device and the device cost are increased.

U.S. Pat. No. 6,337,224 discloses an amorphous thin-film solar cell in which a sufficient thickness of an amorphous silicon photoelectric conversion layer is about 0.3 μm. This document also discloses a polycrystalline thin-film amorphous cell in which a polycrystalline silicon photoelectric conversion layer has to be deposited to a thickness of about 3 μm due to relationship with the light absorption coefficient of the layer. Therefore, in a case where a polycrystalline thin-film solar cell is manufactured using a CVD device, the amount of unnecessary deposits that adhere inside the CVD reaction chamber and to the components located inside the reaction chamber per one batch of the CVD device is higher than in a case where an amorphous thin-film solar cell is deposited. As a result, the frequency at which the CVD device has to be cleaned is increased.

Accordingly, U.S. Pat. No. 4,529,474 and Japanese Patent Laid-Open No. 3-157667 suggest to perform chemical cleaning by plasma etching and gas etching using a mixed gas of CF₄ and O₂ or ClF₃ gas, without disassembling the CVD device. A great advantage of such plasma etching is that the CVD device that has to be cleaned is not necessary to disassemble. However, a significant reaction time is required to remove the adhered dust by the dry etching reaction.

Furthermore, a halogen compound gas that is typically used for plasma etching is expensive. In addition, halogen compound gases require utmost attention in handling because they are typically highly toxic and combustible gases and a certain amount of halogen-containing substances remains in the reaction chamber even after plasma etching. In a case where a halogen-containing deposit remains, if the halogen that has evaporated from the residue is taken in the film deposited after the etching, then properties of the deposited film are greatly degraded.

To resolve this problem, Japanese Patent Laid-Open No. 9-36096 discloses an example of a reaction chamber cleaning method for removing such halogen-containing deposits. More specifically, a method is disclosed by which gas including moisture is introduced in the reaction chamber to induce a chemical reaction with a residual deposit formed by a reaction of a halogen with a wiring material of a semiconductor device. As a result of the chemical reaction, the halogen-containing deposit is decomposed and removed and the halogen is evaporated.

Furthermore, Japanese Patent Laid-Open No. 58-89944 discloses a feature of blowing and removing high-pressure gas in order to remove a dust powder (peeled-off deposit) located inside a reaction furnace.

Japanese Patent Laid-Open No. 2001-131753 discloses a feature of blowing an inactive gas in order to blow off dust that includes a vapor-phase precipitated powder and flakes and has unstably adhered to regions other than a substrate inside a reaction chamber.

As described hereinabove, after the accumulated time of film formation in a CVD device reaches a predetermined level, the useless deposit has to be removed from the inner walls of the reaction chamber or from the surface of components located inside the reaction chamber. Manual disassembling and cleaning of the reaction chamber or dry etching with a halogen compound gas are typical methods used to remove the deposit.

However, the manual method involves time-consuming operations and a high labor cost. When cleaning is performed by dry etching, time can be effectively saved with the method suggested in Japanese Patent Laid-Open No. 9-36096, but unless the dry etching frequency is decreased, further reduction in tact time is difficult to attain.

Dust that includes a vapor-phase precipitated powder and flakes can be removed by blowing gas thereon, as disclosed in Japanese Patent Laid-Open Nos. 58-89944 and 2001-131753. However, it is impossible to remove the deposit that has adhered with a certain strength to the inner walls of the reaction chamber and the surface of components located inside the reaction chamber and is not in the form of vapor-phase precipitated powder and flakes at the time the gas is blown. Furthermore, in some cases, this deposit starts peeling off from the inner walls of the reaction chamber and the surface of components located inside the reaction chamber after a certain time has elapsed. Therefore, the above-described measures can be insufficient.

SUMMARY OF THE INVENTION

The invention has been created to resolve the above-described problems and the invention provides a deposit removal method by which a deposit that has deposited on the inner walls of a reaction chamber and/or the surface of components located in the reaction chamber are removed within a short period by a simple method. Further, the invention provides a deposit removal method that reduces the occurrence of problems associated with contamination caused by the removal process.

The results of a comprehensive study conducted by the inventor to resolve the above-described problems demonstrated that a deposit that has deposited on the inner walls of a reaction chamber and/or the surface of components located in the reaction chamber can be easily stripped by exposing the deposits to a specific atmosphere. This finding led to the creation of the invention

The invention of the present application essentially relates to a deposit removal method including: a first process of stripping at least part of a deposit that has deposited on inner walls of a reaction chamber and/or a surface of components located inside the reaction chamber where a deposited film is formed from the inner walls of the reaction chamber and/or the surface of components located inside the reaction chamber; and a second process of physically removing the stripped deposit.

With the deposit removal method in accordance with the invention, the number of times the reaction chamber is manually disassembled and cleaned or dry etched with a halogen compound gas can be reduced or these operations can be eliminated.

The downtime and tact time of the device can be reduced, yield can be increased, and the device operation efficiency can be increased.

When the deposit removal process is implemented, quality degradation of the deposited film when the deposition is thereafter restarted can be inhibited.

By introducing gas including less water than the first gas, or gas containing no water at all into the reaction chamber as the second gas, it is possible to remove the deposit that has been stripped in the first process. At the same time, the reaction chamber can be dried and protected from corrosion. The resultant effect is that contamination is reduced and film formation process following the deposit removal process can be smoothly advanced.

By introducing gas including fewer oxygen molecules than the first gas or gas containing no oxygen molecules at all into the reaction chamber as the second gas, it is possible to remove the deposit that has been stripped in the first process. At the same time, the reaction chamber can be prevented from unnecessary oxidation. The resultant effect is that contamination is reduced and film formation process following the deposit removal process can be smoothly advanced.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating a plasma CVD device that enables the implementation of the deposit removal method in accordance with the invention.

FIG. 2 is a schematic block diagram illustrating a plasma CVD device that enables the implementation of the deposit removal method in accordance with the invention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a schematic block diagram that illustrates a plasma CVD device used in embodiments of the invention. The best mode for carrying out the invention will be explained below with reference to FIG. 1.

In the plasma CVD device shown in FIG. 1, a substrate-supporting electrode 102 and a counter electrode (gas blow-out electrode) 104 are disposed opposite each other inside a reaction chamber 101 where a deposited film is formed. The substrate-supporting electrode 102, which is connected to the ground, supports the substrate 103 and contains inside thereof a heater (not shown in the figure) for heating the substrate 103 to a predetermined temperature. A film-forming starting material gas is blown out as a shower on the substrate 103 through a gas blow-out surface 104 a including a large number of orifices when a valve V101 is opened. Furthermore, a vacuum pump (not shown in the figure) is provided downstream of a gas release piping 106 for film formation, and when the valve V102 is opened, evacuation is performed and the reaction chamber 101 is maintained at a desirable pressure by a pressure regulating valve (not shown in the figure) located between the vacuum pump and the valve V102. The gas blow-out electrode 104 is electrically insulated from the reaction chamber 101 by an insulating member 107 and electrically connected via a matching box (not shown in the figure) to a high-frequency power source 105 and a DC power source (not shown in the figure).

In such a CVD device, high-frequency power is applied from the high-frequency power source 105 electrically connected to the gas blow-out electrode 104, while supplying a starting material gas including, for example, a silane gas into the depressurized reaction chamber 101. As a result, plasma induced by a glow discharge is generated between the substrate 103 and the gas blow-out electrode 104. The starting material gas is decomposed by the plasma reaction, and the desired thin silicon film is formed on the substrate 103 heated to the predetermined temperature. A substrate removal chamber (not shown in the figure) that can be evacuated is disposed adjacently to the reaction chamber 101. After a film has been formed, the substrate is transferred from the reaction chamber 101 to the substrate removal chamber, the substrate removal chamber alone is used as a vent, and the substrate is replaced, thereby making it possible to form films continuously, while maintaining vacuum in the reaction chamber 101.

As the number of film formation cycles increases, the thickness of useless deposits that are deposited on inner walls of the reaction chamber 101 and internal components of the reaction chamber, such as the gas blow-out electrode 104, by the CVD reaction also increases. A method for removing the useless deposits will be described below.

After film formation, the valve V101 is closed, the supply of the film-forming gas is stopped, the inside of the reaction chamber 101 is evacuated, the valve V102 is closed, and the reaction chamber 101 is vacuum sealed. A gas piping 108 is a nitrogen gas piping, and where the valves V103 and V104 are opened, pure water 111 located inside a tank 110 passes through a first gas piping 109, and the gas that includes water due to the bubbling effect is introduced in the reaction chamber 101. As a result, the deposit that has been deposited on the inner walls of the reaction chamber 101 and components located inside the reaction chamber is stripped by the water-containing gas.

The valves V103 and V104 are closed when the pressure in the reaction chamber 101 reaches about 0.05 MPa. A valve V105 is then opened and the water-containing gas located in the reaction chamber 101 is released via a cleaning piping 113 by a cleaning pump (not shown in the figure) disposed downstream of the cleaning release piping 113.

The valves V106 and V107 are then opened, while performing evacuation with the cleaning pump (not shown in the figure). As a result, dry gas (gas that contains less water than the aforementioned water-containing gas, or contains no water at all) is introduced via the second gas piping 112 into the reaction chamber 101. The deposit stripped from the inner walls of the reaction chamber 101 and components located inside the reaction chamber is then physically blown off. The deposit is trapped in a trap 114 via the cleaning release piping 113.

The inventor has discovered that by introducing water-containing gas into the reaction chamber, it is possible to strip forcibly and efficiently the deposit that has deposited on the inner walls of the reaction chamber and/or the surface of components located inside the reaction chamber. Therefore, it is possible to remove effectively and in advance not only the deposit that has peeled off by itself, but also the deposit that will start peeling after a predetermined time. This effect can be explained as follows. Due to adhesion or permeation of water to the deposit, certain changes occur in internal stresses, thereby causing the deposit to peel off from the internal walls of the reaction chamber and/or the surface of components located inside the reaction chamber.

Rare gases, nitrogen, and hydrogen can be used as the first and second gases, and the effect of the invention can be also obtained in a case where the first gas is caused to flow under evacuation. The inventor has discovered that, from the standpoint of enhancing the stripping of deposit, it is preferred that a water-containing gas be introduced into the reaction chamber and sealed therein for 1 sec or longer, even more preferably for 1 min or longer, in the first process.

Examples of the method for introducing water include gasification by bubbling or heating, introduction of water into a carrier gas by gasification with ultrasound, and direct gasification and introduction of water.

The inventor has discovered that in a case where the deposited film is amorphous silicon, when the temperature of the first gas is 25° C. and the temperature of the inner walls of the reaction chamber is 25° C., the effect of the invention can be confirmed by introducing water at a ratio per unit volume of the deposit of equal to or higher than 15 g/m³ into the reaction chamber. The specific value may be appropriately adjusted correspondingly to the temperature of the first gas and temperature of the inner walls of the reaction chamber.

According to the invention, no limitation is placed on the amount of water introduced in the reaction chamber, but where liquefaction occurs inside the reaction chamber, an unnecessary chemical reaction can occur with components located inside the reaction chamber or residues. Therefore, a range in which no liquefaction occurs is preferred.

Another preferred embodiment of the invention will be explained below.

FIG. 2 is a schematic block diagram illustrating a plasma CVD device used in this embodiment of the invention.

In the plasma CVD device shown in FIG. 2, a substrate-supporting electrode 202 and a counter electrode (gas blow-out electrode) 204 are disposed opposite each other inside a reaction chamber 201 where a deposited film is formed. The substrate-supporting electrode 202, which is connected to the ground, supports a substrate 203 and contains inside thereof a heater (not shown in the figure) for heating the substrate 203 to a predetermined temperature. A film-forming starting material gas is blown out as a shower on the substrate 203 through a gas blow-out surface 204 a including a large number of orifices when a valve V201 is opened. Furthermore, a vacuum pump (not shown in the figure) is provided downstream of a gas release piping 206 for film formation. When the valve V202 is opened, evacuation is performed and the reaction chamber 201 is maintained at a desirable pressure by a pressure regulating valve (not shown in the figure) located between the vacuum pump and the valve V202. The gas blow-out electrode 204 is electrically insulated from the reaction chamber 201 by an insulating member 207 and electrically connected via a matching box (not shown in the figure) to a high-frequency power source 205 and a DC power source (not shown in the figure).

In such a CVD device, high-frequency power is applied from a high-frequency power source 205 electrically connected to the gas blow-out electrode 204, while supplying a starting material gas including, for example, a silane gas into the depressurized reaction chamber 201. As a result, plasma induced by a glow discharge is generated between the substrate 203 and the gas blow-out electrode 204. The starting material gas is decomposed by the plasma reaction, and the desired thin silicon film is formed on the substrate 203 heated to the predetermined temperature. A substrate removal chamber (not shown in the figure) that can be evacuated is disposed adjacently to the reaction chamber 201. After a film has been deposited, the substrate is transferred from the reaction chamber 201 to the substrate removal chamber, the substrate removal chamber alone is used as a vent, and the substrate is replaced, thereby making it possible to form films continuously, while maintaining vacuum in the reaction chamber 201.

As the number of film formation cycles or the accumulated time of film formation increase, the thickness of useless deposits that are deposited on inner walls of the reaction chamber 201 and internal components of the reaction chamber, such as the gas blow-out electrode 204, by the CVD reaction also increases. A method for removing the useless deposits will be described below.

After film formation, the valve V201 is closed, the supply of the film-forming gas is stopped, the inside of the reaction chamber 201 is evacuated, the valve V202 is closed, and the reaction chamber 201 is vacuum sealed. Where a valve V203 is opened, oxygen gas is introduced via an oxygen gas piping 208 into the reaction chamber 201. As a result, the oxygen gas strips the deposit that has deposited on the inner walls of the reaction chamber 201 and surface of components located inside the reaction chamber.

The valve V203 is closed when the pressure in the reaction chamber 201 reaches about 0.02 MPa, the valve V204 is opened, and the gas including oxygen molecules and located in the reaction chamber 201 is released via a cleaning piping 210 by a cleaning pump (not shown in the figure) disposed downstream of the cleaning release piping 210.

A valve V205 is then opened as evacuation is performed with the cleaning pump (not shown in the figure). As a result, nitrogen gas having a purity of equal to or higher than 99.9999% and an oxygen molecule concentration of equal to or lower than 0.1 ppm is introduced via a nitrogen gas piping 209 into the reaction chamber 201, and the deposit stripped from the inner walls of the reaction chamber 201 and surface of components located inside the reaction chamber is physically blown off. The deposit is trapped in a trap 211 via the cleaning release piping 210.

The inventor has discovered that by introducing gas including oxygen molecules into the reaction chamber, it is possible to strip forcibly and efficiently the deposit that has deposited on the inner walls of the reaction chamber and/or the surface of components located inside the reaction chamber. This effect can be explained as follows. Due to adhesion or permeation of oxygen molecules to the deposits, certain changes occur in internal stresses, thereby causing the deposit to peel off from the internal walls of the reaction chamber and/or the surface of components located inside the reaction chamber.

Pure oxygen and also gas mixtures of rare gases, and nitrogen with oxygen gas can be used as the first gas, and rare gases and nitrogen can be used as the second gas. Furthermore, the effect of the invention can be also obtained in a case where the first gas is caused to flow under evacuation. From the standpoint of enhancing the stripping of deposits, it is preferred that the gas including oxygen molecules be introduced into the reaction chamber and sealed therein for 1 sec or longer, even more preferably for 1 min or longer, in the first process.

The inventor has discovered that in a case where the deposited film is amorphous silicon, when the temperature of the first gas is 25° C. and the temperature of the inner walls of the reaction chamber is 25° C., the effect of the invention can be confirmed by introducing oxygen molecules at a ratio per unit volume of the deposit of equal to or higher than 20 g/m³ into the reaction chamber. The specific value may be appropriately adjusted correspondingly to the temperature of the first gas and temperature of the inner walls of the reaction chamber.

In accordance with the invention, no limitation is placed on the amount of oxygen molecules introduced in the reaction chamber, but in a case where easily oxidizable components are present inside the reaction chamber, the amount of oxygen molecules may be appropriately adjusted to avoid unnecessary oxidation. Furthermore, where the temperature inside the reaction chamber is high, oxidation can be accelerated. Therefore, in order to avoid unnecessary oxidation, the concentration of oxygen molecules may be adjusted or the first gas including oxygen molecules may be introduced after the temperature inside the reaction chamber has sufficiently decreased.

EXAMPLE 1

An example of depositing a polycrystal thin-film solar cell by using a plasma CVD device shown in FIG. 1 will be described below.

Silane gas and hydrogen gas are used as the starting material gases for film formation, and VHF 60 MHz is used as a high-frequency power source at a substrate temperature of 200° C. and a film formation pressure of 1300 Pa. A DC component from a DC power source (not shown in the figure) is superimposed on the high frequency via a matching box (not shown in the figure), and a polycrystalline silicon solar cell with a thickness of a photoelectric conversion layer of about 3 μm is deposited on the substrate 103.

In such a system, the DC voltage value is usually controlled to obtain a constant DC current value, but the thickness of useless amorphous silicon deposit that has deposited on the gas blow-out electrode 104 increases and the resistance component also increases as the accumulated number of film formation cycles increases. As a result, the superimposed DC voltage value also has to be increased.

In this example, in the first cycle of film formation, a DC voltage value necessary to obtain a DC current of −2 A is −100 V, whereas in the fortieth cycle the necessary DC voltage value increases to −300 V. The increase in the DC voltage value means the increased probability of spark generation between the gas blow-out electrode 104 and substrate 103. Furthermore, once a spark occurs, it causes significant damage to the reaction chamber 101. Therefore, the deposits present on the gas blow-out electrode 104 have to be removed when the DC voltage value becomes equal to or higher than a predetermined value.

In the present example, a heater (not shown in the figure) incorporated in the substrate-supporting electrode 102 is turned OFF after 40 cycles of film formation, the temperature inside the reaction chamber 101 is decreased to room temperature, and the reaction chamber 101 is evacuated to a pressure equal to or lower than 5 Pa and sealed. As a first process for stripping the deposits, the valves V103 and V104 are opened, and nitrogen including water is introduced into the reaction chamber 101 to a pressure of 0.07 MPa and allowed to stay therein for 5 min. The valves V103 and V104 are then closed, the valve V105 is opened, and gas including water is released. As a second process of removing the deposits physically, the valve V107 is opened and closed several times in a state in which the valve V106 is open, and dry nitrogen is introduced into the reaction chamber 101. The deposits present on the reaction chamber 101 and gas blow-out electrode 104 are then blown off for collection with the trap 114.

Where a polycrystalline silicon solar cell is again film-formed under the above-described film formation conditions upon completion of the deposit removal process, the DC voltage value returns to −100 V. Furthermore, the obtained polycrystalline silicon solar cell has good properties. This result means that the deposit present on the gas blow-out electrode 104 is removed and water that is contamination is also removed. Where the device is purged upon completion of film formation, the trap 114 is detached, and the inside of the trap 114 is observed, the presence of deposit trapped therein can be confirmed.

Thus, with the deposit removal method in accordance with the invention, it is not necessary to perform dry etching or disassembling of the reaction chamber 101 and the tact time, yield, and device operation efficiency can be improved.

COMPARATIVE EXAMPLE 1

A total of 40 polycrystalline thin-film solar cells are film-formed by a procedure similar to that of Example 1 by using the CVD device shown in FIG. 1. Then nitrogen sufficiently dried to a water content of about 540 ppb, based on percents by volume, is introduced to a pressure of 0.07 MPa from the gas blow-out electrode 104 into the reaction chamber 101 as the first gas for removing the deposits. Furthermore, nitrogen is introduced as the second gas from the second gas piping 112 in the same manner as in Example 1.

After a total of 40 cycles of deposition, the DC voltage value necessary to obtain a DC current of −2 A is −300 V, as in Example 1, but the DC voltage value after the deposit removal process is −250 V.

This is apparently because the deposits are not removed by dried nitrogen introduced in the first process, but the amount of resistance component is decreased because the deposits are somewhat blown off in the second process. The amount of deposits trapped in the trap is about ¼ that of Example 1.

The above-described Comparative Example 1 also clearly demonstrates the deposit stripping effect produced by introduction of water-containing gas as the first gas.

The variation amount of internal stresses necessary to strip the deposit also differs depending on the film thickness of deposit, film structure, film density, and temperature of the internal walls of the reaction chamber. Therefore, the amount of water necessary for stripping also varies depending on these conditions. As a result, the moisture content in the first gas and the sealing pressure during introduction of the first gas into the reaction chamber that are necessary to strip the deposit also vary. Accordingly, the moisture content necessary to obtain the effect of the invention is preferably found in advance by a preliminary test or the like.

EXAMPLE 2

An example of forming a polycrystal thin-film solar cell by using a plasma CVD device shown in FIG. 2 will be described below.

Silane gas and oxygen gas are used as the starting material gases for film formation, and VHF 60 MHz is used as a high-frequency power source at a substrate temperature of 200° C. and a film formation pressure of 1300 Pa. A DC component from a DC power source (not shown in the figure) is superimposed on the high frequency via a matching box (not shown in the figure), and a polycrystalline silicon solar cell with a thickness of a photoelectric conversion layer of about 3 μm is film-formed on the substrate 103.

In such a system, the DC voltage value is usually controlled to obtain a constant DC current value. However, the thickness of useless amorphous silicon deposit that is deposited on the gas blow-out electrode 204 increases and the resistance component also increases as the accumulated number of film formation cycles increases. As a result, the superimposed DC voltage value also has to be increased.

In this example, in the first cycle of film formation, a DC voltage value necessary to obtain a DC current of −2 A is −100 V, whereas in the fortieth cycle the necessary DC voltage value increases to −300 V. The increase in the DC voltage value means the increased probability of spark generation between the gas blow-out electrode 204 and substrate 203. Furthermore, once a spark occurs, it causes significant damage to the reaction chamber 201. Therefore, the deposit present on the gas blow-out electrode 204 has to be removed when the DC voltage value becomes equal to or higher than a predetermined value.

In the present example, a heater (not shown in the figure) incorporated in the substrate-supporting electrode 202 is turned OFF after 40 cycles of film formation, the temperature inside the reaction chamber 201 is decreased to room temperature, and the reaction chamber 201 is then evacuated to a pressure equal to or lower than 5 Pa and sealed. As a first process for stripping the deposits, the valve V203 is opened, and oxygen gas is sealed in the reaction chamber 201 to a pressure of 0.02 MPa. The valve V203 is then closed, the valve V204 is opened, and gas including oxygen molecules is released. As a second process of removing the deposits physically, the valve V205 is opened and closed several times in a state in which the valve V204 is open, and nitrogen is introduced into the reaction chamber 201. The deposits present on the reaction chamber 201 and gas blow-out electrode 204 are then blown off for collection with the trap 211.

Where a polycrystalline silicon solar cell is again film-formed under the above-described film formation conditions upon completion of the deposit removal process, the DC voltage value returns to −100 V. Furthermore, the obtained polycrystalline silicon solar cell has good properties. This result means that the deposit present on the gas blow-out electrode 204 is removed. Where the device is purged upon completion of film formation, the trap 211 is detached, and the inside of the trap 211 is observed, the presence of deposit trapped therein can be confirmed.

Thus, with the deposit removal method in accordance with the invention, it is not necessary to perform dry etching or disassembling of the reaction chamber 201 and the tact time, yield, and device operation efficiency can be improved.

COMPARATIVE EXAMPLE 2

A total of 40 polycrystalline thin-film solar cells are film-formed by a procedure similar to that of Example 2 by using the CVD device shown in FIG. 2. Then nitrogen having a purity of equal to or higher than 99,9999% and an oxygen molecule concentration of equal to or less than 0.1 ppm is introduced to a pressure of 0.02 MPa from the gas blow-out electrode 204 into the reaction chamber 201 as the first gas for removing the deposits. Furthermore, nitrogen is introduced as the second gas from the second gas piping 212 in the same manner as in Example 2.

After a total of 40 cycles of film formation, the DC voltage value necessary to obtain a DC current of −2 A is −300 V, as in Example 1. The DC voltage value after the deposit removal process is −250 V.

This is apparently because the deposits are not stripped by pure nitrogen introduced in the first process, but the amount of resistance component is decreased because the deposits are somewhat blown off in the second process. The amount of deposits trapped in the trap is about ¼ that of Example 1.

The above-described Comparative Example 2 also clearly demonstrates the deposit-stripping effect produced by introduction of gas including oxygen molecules as the first gas.

In the present example, the stripping is performed by sealing oxygen molecules in the reaction chamber 201, but the variation amount of internal stresses necessary to strip the deposits also differs depending on the film thickness of deposits, film structure, film density, and temperature of the internal walls of the reaction chamber. Therefore, the amount of oxygen molecules necessary for stripping also varies depending on these conditions. As a result, the concentration of oxygen molecules in the first gas and the sealing pressure during introduction of the first gas into the reaction chamber that are necessary to strip the deposits also vary. Accordingly, the amount of oxygen molecules necessary to obtain the effect of the invention is preferably found in advance by a preliminary test or the like.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2008-163634, filed Jun. 23, 2008, which is hereby incorporated by reference herein in its entirety. 

1. A deposit removal method comprising: a first process of stripping at least part of a deposit that has deposited on inner walls of a reaction chamber and/or a surface of components located inside the reaction chamber where a deposited film is formed from the inner walls of the reaction chamber and/or the surface of components located inside the reaction chamber; and a second process of physically removing the stripped deposit.
 2. The deposit removal method according to claim 1, wherein the first process is a process of introducing a first gas including water into the reaction chamber.
 3. The deposit removal method according to claim 2, wherein the second process is a process of introducing a second gas that contains less water than the first gas or contains no water at all into the reaction chamber.
 4. The deposit removal method according to claim 1, wherein the first process is a process of introducing a first gas including oxygen molecules into the reaction chamber.
 5. The deposit removal method according to claim 4, wherein the second process is a process of introducing a second gas that contains fewer oxygen molecules than the first gas or contains no oxygen molecules at all into the reaction chamber.
 6. The deposit removal method according to claim 1, wherein the deposit that has deposited on inner walls of a reaction chamber and/or a surface of components located inside the reaction chamber is a silicon film. 